Black screen on MacBook since Mountain Lion

2007 (per unibody) Macbook Pro with Hi-Res 1920x1200 screen and 4 gig of ram

Since upgrading to Mountain Lion on Wednesday if the machine goes to sleep, restarts, it's shutdown or even the lid closed it will not rebook for several hours. It doesn't matter if you have used it for 1 minute or 8 hours it just reboots to a black screen, fan running full speed and no keyboard illumination.

Te only way to get it to reboot is leave it shutdown for several hours and then it's fine again unless you do any of the above.

I have tried resetting SMC an PRAM but still the same.

Anyone got any ideas or do I need to go back to Lion?

I look forward to your responses and hopefully a simple fix..

Bob